Color Index Method for New Alternative Determining Fajr Time
Hendro Setyanto (a*), Hendra Agus Prastyo (b)

a) Imahnoong Observatory and Planetarium, Lembang, Indonesia
*setyanto[at]gmail.com
b) Department of Atmospheric and Planetary Sciences, Sumatera Institute of Tecnology, Jalan Terusan Ryacudu, Lampung Selatan 35365, Indonesia

Abstract

Astronomers use the color index method to determine a star^s temperature by subtracting its magnitude in different filters. A lower value of the star^s color index means the temperature is hotter than the higher value of a color index. This method helps astronomers classify stars^ temperatures in the universe. Inspired by this method, we use it as a new alternative to determining fajr time from imagery data to find the reddening effect of the sky. The images of fajr were taken by using a DLSR camera and then split into three bands, called R (Red), G (Green), and B (Blue). These images of fajr were taken at National Observatory in Timau-NTT. The color index method is used to find the value R-G and R-B for each image. To see when the reddening effect of the eastern sky begins, the value of the R-G color index compares to the value of the R-B color index. The results show that when the Sun^s altitude is -48 degrees, the value of R-G is 0.40 while the value of R-B is -0.99. The reddening impact on the eastern sky begins when the Sun^s altitude is -20 degrees.

Keywords: Color Index- Fajr- Reddening Effect

Topic: EARTH, ATMOSPHERIC, AND SPACE SCIENCE